1. Field of the Invention
The invention relates to a microphone preamplifier, and in particular, to a low noise design that reduces interferences caused by thermal noise and flicker noise.
2. Description of the Related Art
A typical microphone has a capacitor for sensing the sound pressure. As the capacitance of the capacitor varies in proportional to the sound pressure, the sound is thereby converted to a voltage signal. To maintain sensibility of the microphone, it is important not to load the microphone capacitance with a resistive load, because a resistive load will discharge the capacitor and thereby ruin the linear dependency of the sound pressure.
A preamplifier is therefore required to resolve the consequences. The preamplifier is generally configured with a high input resistance, transforming the output voltage from the microphone to post stages without loss of the sensibility. The preamplifier is typically connected physically very close to the capacitor, within a distance of very few millimeters or fractions of millimeters.
Conventionally, a preamplifier comprises at least two operational amplifiers and a plurality of resistors to generate a differential outputs based on a single ended microphone input signal. Thermal noise and flicker noise are unavoidably generated from those components, affecting the signal quality of the differential outputs. As the modern technology develops, requirement for compact size and power efficiency also increase rapidly. The conventional preamplifier turns out to be inadequate because it consumes significant power to suppress the undesirable noises. It is therefore desirable to propose an improved architecture of a microphone preamplifier.